(1) Field of the Invention
The present invention relates to an injection type semiconductor laser device. More particularly, the invention relates to an injection type semiconductor laser device having a structure effective for stabilizing the lateral mode and a method for manufacturing the same.
(2) Description of the Prior Art
In injection type semiconductor laser devices, direct current oscillation at temperatures approximating room temperature has been made possible by adoption of a GaAs-Al.sub.x Ga.sub.1-x As double-hetero structure.
Recently, improvements of optical fibers have been advanced and optical fibers having a much reduced light transmission loss at a wave length of about 1 to about 1.3 .mu.m have been developed. Accordingly, a semiconductor laser of this wave length region is very valuable as a light source for optical communication using such optical fibers.
Various proposals have heretofore been made in connection with stabilization of the transverse mode in injection type semiconductor laser devices. For example, there can be mentioned a structure published by Aiki et al. in Applied Physics Letters, Vol. 30, No. 12, pages 649-651 (1977). More specifically, there is proposed a semiconductor laser device having a double-hetero structure, which comprises a first semiconductor layer having a so-called active region and second and third semiconductor layers having a band gap larger than that of the first semiconductor layer, which are disposed on both the sides of the first semiconductor layer, respectively. In this semiconductor laser device, the thickness is changed stepwise in the second semiconductor layer and a fourth material layer having the complex refractive index changed with respect to laser beams is disposed at least on the outside of the thickness-reduced region of the second semiconductor layer. The transverse mode is stabilized by the effect of bleeding of laser beams in the fourth material region. Indeed, this proposal is effective for an injection type semiconductor laser device having a GaAs-Al.sub.x Ga.sub.1-x As double-hetero structure, but in case of a GaAs-Al.sub.x Ga.sub.1-x As type semiconductor laser device, long-wave length oscillation, especially at a wave length of 0.9 to 1.7 .mu.m, is hardly possible theoretically. As the long-wavelength oscillation semiconductor laser device, there can be mentioned an InP-Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y type semiconductor laser device. If only the above-mentioned structure in which the thickness is changed stepwise in the second semiconductor layer is adopted for this semiconductor laser device, no sufficient stabilization of the lateral mode can be attained. The reason is that the InP layer fails to sufficiently absorb laser beams from the Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y layer as the active layer.